1. Field of the Invention
The invention relates to a method for the surface treatment and compressive prestressing by peening of the tops and aerodynamic surfaces of cooled blade tips, which include a cavity in their tops, said blades being hollow and having an internal cooling circuit communicating with the outside via cooling air passage orifices formed, in particular, in said tops and said aerodynamic surfaces.
2. Technical Background
A cooled blade, for example a jet engine turbine blade, is hollow and comprises a root portion for securing the blade to a rotor, and an airfoil portion. The root has a sizeable opening through which a flow of cooling air tapped from a compressor stage is injected. The blade airfoil includes, on a leading edge, on a trailing edge, on flanks and on the top, small orifices which allow the cooling air, which is heated-up as it flows inside the blade, to be removed so as to be replaced with fresh cooling air, and which allow the conveying of this heated-up cooling air onto an exterior surface of the blade to isolate it from combustion gases of the jet engine. These orifices, which may be cylindrical or may be in the form of slots, have relatively small transverse dimensions, at most equal to 1 mm. Furthermore, the aerodynamic surfaces comprising the leading edge, the trailing edge, and the flanks are generally extended at the top of the blade by an exterior wall forming, at the top of the blade, a cavity which is open toward the top.
Turbine blades in operation are subjected to considerable centrifugal forces and to high thermal stresses. Hence, it is necessary to compressively prestress the external surfaces of the aerofoils so as to delay the onset of cracks detrimental to the integrity and life of these blades, and particularly the surfaces at the top of the blades which are subjected to the greatest centrifugal forces.
This compression is also needed in the case of blades whose tops have been repaired. Repairing the top of a blade includes the steps of building up the uneven regions using weld metal, in practice by TIG electric arc welding. This build-up is followed by machining to reconstruct the shape of the blade, and then by peening, by blasting beads or shot, so as to compress the surface of the built-up regions and prevent the appearance of cracks at the welds during the subsequent heat treatments and when the blade is in use.
When the top of the blade has a cavity open toward the top, the inside of a wall delimiting this cavity has also to be peened.
At the present time, the surfaces of the blade tips are compressively prestressed by peening using beads ejected by air jet peening nozzles. These nozzles are able to project only beads the diameter of which is smaller than 1 mm, and typically between 0.1 and 0.5 mm. It is therefore necessary to plug the blade cooling orifices, prior to peening, to prevent the beads from entering them. The top of the blade is then offered up to one or more air jet peening nozzles and the nozzles are moved in order to make the peening uniform. After the peening operation, the cooling orifices have to be unplugged.
It is also possible for peening to take place without plugging the orifices during the operation. In such a case, it is then necessary, for example by dissolving them in acid, to remove any beads which may remain in the cooling orifices or may have entered the interior cavity of the blade.
Peening with nozzles takes a great deal of time since the beads are of small diameter they have low kinetic energy. Furthermore, the small diameter of the beads. leads to damage to the surface finish of the blade.
During peening, the nozzle has to be moved in various orientations, so as to reach the entire peripheral wall of the top of the blade and also the interior wall of the upwardly open cavity when there is one. When peening is performed by hand, it is, by nature, not very repeatable. When peening is automated, it entails control over numerous parameters, and is therefore difficult to control and to repeat.
It is also known practice for peening to be carried out using flappers equipped with beads. This method, known as xe2x80x9cflapper peeningxe2x80x9d, includes the step of using a small spindle comprising, at its end, strips of plastic, to the end of which beads are bonded. Peening is performed by rotating the spindle. As the beads are held on the strips, they do not enter the cooling orifices. The disadvantage with this method is that the tool soon degrades as beads are torn off and the strips become frayed, and that, above all, it is not possible effectively to peen the interior of the open cavity at the top of the blade.
It is an object of the invention to propose a method for peening blade tips which makes it possible to repeatedly create identical prestresses in a relatively short peening time.
It is another object of the invention to propose a method for peening blade tips which eliminates the operations of plugging and unplugging the cooling air passage orifices before and after the peening operation respectively.
According to the invention, there is provided a method for the surface treatment and compressive prestressing by peening of the tops and aerodynamic surfaces of cooled blade tips which include a cavity in their tops, said blades being hollow and having an internal cooling circuit communicating with the outside via cooling air passage orifices formed in said tops and said aerodynamic surfaces, said method comprising the steps of providing a sonotrode having a vibratory surface and ultrasonic means for vibrating said vibratory surface, placing a plurality of beads on said vibratory surface of said sonotrode, said beads having a diameter greater that that of said cooling air passage orifices, providing means for supporting at least one blade, said supporting means and said vibratory surface of said sonotrode together at least partly defining a sealed chamber containing at least one blade tip, and operating said ultrasonic means to vibrate said vibratory surface of said sonotrode whereby said beads are mobilized in said chamber to conduct ultrasonic peening of said at least one blade tip.
According to a preferred embodiment, said beads have a diameter of between 0.8 and 5 mm.
Since the beads have a diameter greater than that of the cooling air passage orifices, it is not necessary to carry out the operations of plugging and unplugging these orifices, and the increased kinetic energy of the beads relative to the beads of the prior art allows the peening time to be reduced.
Furthermore, the mobilized plurality of beads moving in random directions strike the surfaces of the blade tip at varying angles, and this improves the surface finish by comparison with beads projected by a nozzle in a predominant direction, this being especially the case since the beads used in the method according to the invention have a diameter greater than the diameter of the beads that can be projected by a nozzle.
Finally, all the external surfaces of the blade tip exposed in the chamber are subjected at the same time to the impacts of the beads, and this considerably reduces the risk of blade tip deformation.
The invention also provides an apparatus for carrying out the method according to the invention, said apparatus comprising a blade support plate having at least one through-orifice, the cross section of which matches the cross section of the end of a blade aerofoil, a frame, on one face of which said support plate can be fixed, and a sonotrode, a vibratory surface of which is capped by said support plate fixed to said frame, said vibratory surface of said sonotrode being arranged facing said support plate.
According to the invention, there is also provided a method for repairing blades, said method comprising the steps of building up uneven regions of said blade with weld metal, machining said blade to reconstitute the shape of the blade, and peening said blade according to the method of claim 3 so as to prevent the appearance of cracks at said welds during subsequent heat treatments and when said blade is in use.